Fibrous sheet covered plywood



'March 7, D G, BIRMINGHAM 2,343,740

EIBRous SHEET covEEED PLYwooD Filed Sept. 1e, i940 Patented Mar. 7, 1944y FIBROUS SHEET COVERED PLYWOOD v Donald G. Birmingham, Aberdeen, Wash. assignor to Harbor Plywood Corporation, Hoqulam, Wash., a corporation of Delaware .Application September 18, 1940, Serial No. 357,196

11 Claims.

This invention relates to wood veneer covered with a matted fibrous sheet and includes a method of making such laminated products.

More specifically the invention relates to plywood panels having the broad faces thereof covered with a resin impregnated loosely matted pulp sheet integrally bonded to the wood and presenting finished smooth surfaces on the panels The invention also specifically provides a method of making paper covered plywood which eliminates wood grain swelling and results in the complete masking of all wood imperfections.

Commercial plywood panels must have at least one broad face thereof presenting a smooth surface free from'pitch pockets, knotholes, and other imperfections. The surface veneers for such panels must therefore be carefully graded and matched. Only a very small percentage of the veneer sheets'peeled from a log are useful as surface veneers in the plywood panels. A plywood mill therefore rapidly accumulates a surplus of imperfect veneer sheets. 'Ihese sheets are used to form cores or inner veneers in the panels.

Due to the increasing scarcity of good peeler logs from which plywood veneers are made, more and more of the imperfect veneer-sheets are being accumulated and, in order to use up these imperfect sheets, it is now necessary to select the best of them for surface sheets and patch up the imperfections in the best manner possible. The

Apatching operations are time-consuming, expensive, and the nished panels will always contain somewhat visible patches.

The present invention now provides a covering for imperfect surface veneers of plywood panels which completely masks all imperfections and produces a smooth, uniform finish on the panels. According to the invention, fibrous sheets such as ground wood pulp sheets, asbestos sheets, or any other loosely matted fibrous webs are impregnated with a thermosetting or thermoplastic binder. The impregnated sheets are next dried and coated on one face thereof with a water solution of a thermosetting or thermoplastic binder. The coated face is then united with the wood veneer by heat and pressure. If a, multi-ply panel is being made, the cores and inner wood .veneers of the pane1 are united to the surface veneers at -the same time that the fibrous sheets are united to the surface veneers.

It is, then, an object of this invention to provide plywood panels with finished paper surfaces completely masking all imperfections in the wood.

A further object of the invetnion is to utilize imperfect wood. veneers 'in the making of plywood panels having nished surfaces.

A furtherobject of the invention is to provide a method of covering wood veneers with fibrous webs to completely mask all imperfections in the wood.

A specific object of the invention is to mask imperfections in wood veneers by covering the same with a thermosetting or thermoplastic binder-impregnated loosely matted brous sheet.

A specific object of the invention is to provide a method of masking imperfections in wood veneers covered with fibrous sheets by coating'an aqueous binder solution only on the fibrous sheet to prevent grain raising of the wood.

Other and further objects of the invention will become apparent from the following specili-` cation including the description of the annexed sheet of drawings, in which:

Figure 1 is a diagrammatic view illustrating the method of making paper covered plywood panels according to this invention.

Figure 2 is a diagrammatic view illustrating the hot pressing operation employed in the method of this invention.

Figure 3 is a progressive top plan view of a plywood panel made according to this invention illustrating thevarious layers of the panel.

Figure 4 is an enlarged vertical cross-sectional view taken along the line IV-IV of Figure 3.

Figure 5 is an enlarged exploded cross-sectional view of the constituent elements forming the plywood panel shown in Figures 3 and 4. Y

Figure 6 is an isometric view of a plywood panel according to this invention.

Figure 7 is a fragmentary end elevational view of a wood veneer coated with an aqueous binder and a fibrous covering sheet for the veneer illus,-

trating the manner in which the aqueous binder will swell the soft or summer grain of the Wood.

Figure 8 is a view similar to Figure 7 illustrating the hot pressing of the fibrous sheet onto the coated wood veneer and the resulting imbedding of the sheet into the soft grain of the wood. Figure 9 is a view similar to Figure 8 illustrating the wavy surface obtained on the fibrous sheet when the pressure is released due to expanding of the compressed soft wood grain.

Figure 10 is a plan view of the fibrous sheet covered Wood veneer shown in Figure 9, illustrating the visibility of the wood grain formation I through the fibrous sheet.

As shown in Figure 1 of the drawing, a dry continuous sheet of fibrous pulp lll is unreeled from a roll Il, passed over a guide roll l2 and imder a guideroll Il immersed in a tank Il confrom the roll i3 over the roll Il to cover the entire suction area defined by the suction head i1. It is next passed under another guide roll Il and then upwardly between a pair of squeeze rolls Il disposed above the tank Il. After passage through the nip of the squeeze rolls, the web is fed into a drier 20.

The suction head Il is evacuated so as to draw the solution I5 through the entire thickness of the sheet il. The sheet is thus thoroughly impregnated with the solution il and excess solution is squeezed out of the sheet by the rolls Il.

The initial sheet i is preferably relatively dry, containing in the nelghborhood'of only about 1% moisture, and is prepared from pulp that is about 90% ground wood. The fibers in the sheet have not been refined as in the case of a finished paper sheet, but are merely felted or loosely matted together to the usual extent obtained on wet machines" in the manufacture of pulp. The sheet is not calendered so that the bers will content of the sheet can be partially replaced with sulphite pulp or, if a high quality sheet is desired. a relatively pure sulphite liner can be formed on one of the surfaces of the sheet so as to improve the finish thereof.

In place of wood pulp, various other vegetable fibers may be employed. Inorganic fibers such as asbestos may be used in sheet form.

The preferred impregnating medium l5 for the vpulp sheet is an aqueous solution of a cresylic acid-formaldehyde partial condensation product having a viscosity of not over 1,000 centipoises. The thermosettlng cresylic acid-formaldehyde partial condensation product can be prepared as described in the James V. Nevin United States Letters Patent No. 2,150,698 to produce an aqueous binder solution. 'I'he resin solution can be diluted with water in any amount necessary to produce a relatively thin solution which will be readily sucked through the sheet by the suction roll It.

In place of the aqueous solution of cresylic acid-formaldehyde partial condensation product, it should be understood that any other aqueous thermosettlng binder can be used. Likewise, thermoplastic natural or artificial aqueous binder dispersions or solutions can be used. If desired,

the mastic types of aqueous glue solutions can be used. However, unless the selected resin or glue is thermosetting the resultant product ,will not be waterproof. Thermosettlng materials such as the phenolic resins will produce waterproof products.

Various plasticizers, hardeners, and natural or synthetic thermoplastic resins can be incorporated into the thermosetting resin solutions to impart special properties thereto'.

The squeeze rolls l! are preferably regulated so as in permit the impregnated sheet to retain from to 30% of dried resin solids based on the dry weight of the pulp sheet.

The drier dries the impregnated pulp sheet to a moisture content of about 7% without, however. setting the binder impregnated therein.

. readily absorb the solution II. The ground wood Temperatures of about l'. are used in the drier.

The dried impregnated sheet is then passed through a cutter 2| which cuts the sheet into lengths 22 which are stacked in front of the cutter. The sheet lengths 22 are sized in accordance with standard plywood panel dimensions so as to completely cover the broad faces of the panel.

Two superimposed sheet lengths 22 are then passed between coating rolls 22 and 24. Films of an aqueous binder solution. such as a cresylic acid-formaldehyde binder solution, are applied to the rolls 23 and 2l from troughs 2l by rollers 25a. These films are deposited onto the outer faces of the superimposed sheet lengths 22 and are preferably applied when the binder solution is hot to insure a good flowing of the binder. The binder solution has the water content thereof regulated so that the coating applied to 1,000 square feet of surface of each impregnated pulp sheet does not contain more than 10 lbs. of water.

For example, the solids content of the solution can be about 30% and the spread can be '1% lbs. of solution on each 1,000 square feet of coated pulp sheet surface. This produces a water content of 5.25 lbs. per 1,000 square feet cf coating. The binder solids content per 1,000 square feet of coating is thus only 2.25 lbs. Such a small amount of binder, however, is sufficient to create a good bond between the pulp sheet and the wood veneer since the binder impregnated in the sheet is active in aiding the bond.

In other words, a minimum amount of binder solution is applied to one face of each impregnated sheet to effect a good bond with a wood veneer sheet. In accomplishing this, the water content of the coating is regulated so that 1,000 square feet of spread coating will not contain more than 10 lbs. of water, and will preferably contain only about 5 lbs. of water. This moisture content of the coating on the pulp sheet is important, as will be hereinafter explained.

One of two coated pulp sheets is then laid on an assembly table (not shown) with the coated face exposed. A stack of wood veneer sheets 2i is provided adjacent the assembly table and one of these wood veneer sheets is disposed on top of the coated surface of the sheet length 22 as indicated by the arrow.

A stack of wood core sheets 21 is also provided adjacent the assembly table and these sheets are fed between coating rolls 28 and 29 which coat both broad faces thereof with an aqueous thermosetting cresylic acid-formaldehyde resin solution applied to the rolls from troughs 30 and 3l by means of regulating rollers 32 and 3l. The binder solution coated onto the core sheet preferably contains from 40 to 45% of dry solids and is preferably applied in a somewhat heated condition so as to be readily coated over the entire broad faces of the cores. The thickness of the coating is regulated to apply from 7l/2 to 25 pounds of dry binder per 1,000 square feet of coated area.

The coated cores 2l are then deposited on top of the veneer 26 and covered with another uncoated veneer 2B as indicated.

The other of the two coated pulp sheets 22 is then mounted on 'top of the top veneer sheet 26 as indicated, with the coated face contacting the veneer.

I'he thus assembled pulp sheets, veneer sheets and cores form a stack Il which is next placed impregnated pulp sheet. At the same time the binder impregnated into the pulp sheety is set into a hard mass materially increasing the 'strength of the brous sheet. Theplatens have polished surfaces and the surfaces of the pulp sheet are thus provided with a smooth, glossy finish.

Pressures of about 175 pounds per square inch of panel surface are used in the hot press when the veneers are.J formed of Douglas r. If the veneers are formed of redwood, lower pressures of persed throughout the body of the pulp sheet as describe'dabove. y

As' shown in Figure, when the pulp sheet 22 is deposited on the coated veneer 26 and the assembly placed betweenthe platens 36 of the hot press, the loose matted fibers of the pulp sheet will be pressed into the soft swollen summer grain as at 22a, thereby becoming embedded inv the wood and compressing the soft grain. After the release of pressure by the platens 36, as shown in Figure 9 of the drawing, the compressed summer around 130 pounds per square inch of panel suru face are used.

After the hot pressing operation the weldedtogether panels are immediately inserted in a humidifier before they cool down. This humidifier will impart a desired moisture content into the panels so that the humidied panels contain b etween 6 to 12% moisture. Such a moisture content will prevent subsequent warping of the panels and will also prevent checking along the glue lines.

The finished panels 40 shown in Figures 3, 4 and 6v are thereby produced. These panels comprise wood cores 4l permanently welded to wood veneers 42 through the thermoset glue lines 43, and paper covers 44 impregnated with a thermoset resin and united to the outer faces of the veneers 42 through thermoset glue lines 45. As

will be noted from Figures 3 and 6, the paperV covers 44 have smooth, glassy surfaces. These surfaces, due to the thermoset binder impregnated therein, are weatherproof, hard, and wearresisting.

As best shown in the exploded view in Figure 5, the panels 40 are initially made from the cores 2 coated on both faces with a thermosetting binder B, the uncoated'veneers 26, and the impregnated pulp sheets 22 having thicker binder coatings B on their inner faces as shown.

In order to insure the complete masking of any imperfections in the veneers 26 it has been found that the coating B should be applied to the pulp sheets 22 instead of to the outer surfaces o f the veneers 26. It has also been found that when the binder coatings B are applied from aqueous solutions or dispersions, the moisture content of the coating B on the pulp sheets 22 should contain less than l0 lbs. of Water per 1,000 square feetof coated surface.

Figures 7 to 10 illustrate what will happen-if the moisturecontent exceeds l0 pounds per 1.000

square feet of coated surface and if the coatingsV B are applied to the veneers 26 instead of to the pulp sheets 22.

In Figure 7, a veneer 26 is coated with an aqueous binder B. The water content of the binder swells the summer or soft grain in the Wood as at 26a. The winter or hard grain 26h does not swell appreciably, thereby producing a wavy surface on the veneer. i

The impregnated pulp sheet 22 does not have a binder coating B thereon, but the binder is disgrain of the wood, now dried by the hot pressing operation, springs back into flush relation with the hard grain, thus producing a flat surface in the veneer 26. However, since the pulp sheet 22 has been embedded in the compressed softgrain and is thickened at the embedded portions, this springing back of the soft grain moves the embedded pulp sheetporti'ons therewith and pro duces a wavy surface 22h in the sheet. As shown in Figure 10, the wavy contour 22h will reproduce the grain formation in the 4veneer 26 and. if any imperfectionasuch as pitch pockets and the like soft -spots are present in the wood veneer, these will also show upA in the surfaceof the panel. u

Thus, according to this invention, the pulp sheets 22 are coated with the binder coating B instead of the wood veneer 26 being coated with the binder coating. As a result, the invention provides paper covered plywood panels with smooth,. glossy surfaces completely masking all imperfections. in the underlying wood structure.

It will, of course, be understood that various' details ofthe invention may be varied through a wide lrange without departing from the principles thereof and it is, thereforanot the purpose to limit the ypatent granted hereon otherwise than necessitated'by the scope of the appended claims.

I claim as my invention:

l. The method of making fibrous web covered wood products which comprises superfically coating a broad face of a brous web with a resinous binder, pressing the coated face against a wooden surface to be. united *thereto and heating the pressed-together surfaces for uniting the members to mask any imperfections in the wood surface.

2. The method of making laminated wood productsv having fibrous web covers thereon which comprises coating a.V fibrous web with an aqueous dispersion of a resinous binder havinga water content such as to produce a superficial water content of le'ss than- 10 pounds of water per 1,000 square feet of coated surface, pressing *the coated surface against a wooden sheet, heating the pressed-together web and sheet to drive the water out of the binder and to permanently unite the fibrous sheet to the wood for compietely masking any imperfections in the wood surfaces. l l

3. The method of making'brous web covered laminated wood products which comprises coating`brous webs with -arr aqueous dispersion of a resinous binder having a water content such as to 4. The method of making pulp covered laminated wood products which comprises superflcially coating one' tace of a loosely matted ilbrous pulp sheet with an aqueous dispersion of a thermosetting resinous binder having a water content such as to effect a superficial water content of less then pounds of water per 1,000 square feet of coated surface, pressing the coated surface against a wooden veneer sheet and heating the assembly to set the resin for permanently uniting the pulp sheet to the wood whereby the pulp sheet will completely mask any imperfections in the wood surface.

5. The method of making fibrous web covered laminated wood products which comprises diffusing a resin forming dispersion through a fibrous web to retain in the web an amount of resin insumcient to saturate the web, drying the fibrous web, coating one face of the web with a resinous binder, pressing the coated face of the web against a wood veneer and heating the assembly to unite the web with the veneer.

' 6.'The method of making fibrous web covered laminated wood products which includes diffusing a thermosetting 'lresin forming dispersion through a fibrous web to retain in the web an amount of resin insumcient to saturate the web, drying the web at temperatures below the setting point of said resin, coating one face of the web with an aqueous dispersion of a thermosetting resinous binder in amounts insuilicient to effect a' superficial water content of more than 10 pounds of water per 1000 square feet of coated surface, pressing the coated surface against a wood -veneer, and heating the assembly to set thermally the diffused resin and the binder for uniting the web to the wood and for hardening the fibrous web.

7. The method of making pulp covered laminated wood products which includes diffusing an aqueous dispersion capable of forming a thermoseting resin through a paper pulp sheet to retain in the sheet an amount of resin insufficient to saturate the sheet, drying the sheet at temperatures below the setting point of the resin, coating one face of the dried sheet with a thicker dispersion of a thermotting resinous binder in amounts insuiilcient to effect a water content of more than 10 pounds of water per 1000 square feet of coated surface, pressing the coated surface against a wood veneer sheet, and heating the pressed-together assembly for thermally setting the binder in the coating and the resin diffused through the sheet.

8. The method of making pulp covered plywood. which comprises diffusing an aqueous resin forming solution of a cresylic acid-formaldehyde condensation product through a loosely matted fibrous sheet to retain in the sheets an amount of resin insufficient to saturate the sheet, drying the sheet at a temperature below the setting point of the condensation product, coating one face of the dried sheet with a thicker aqueous solution of a cresylic acid-formaldehyde binder in amounts insufficient to effect a superficial water content of more than 10 pounds of water per 1000 square feet of coated surface. forming a stack of wood veneers and wood core sheets with thermosetting cresylie acid-formaldehyde binder on the core sheets, covering at least one of the veneer sheets with a coated pulp sheet having said resin forming product diffused therethrough, and hot pressing the assembly to set thermally the eresylic acid-formaldehyde produets for permanently welding the cores, veneers and pulp sheets to produce a plywood panel with a pulp cover completely masking all imperfections in the wood.

9. The method of making pulp covered plywood which comprises passing a pulp sheetthrough an aqueous solution of cresylic acidformaldehyde partial condensation product. sucking the solution completely through the sheet to impregnate the sheet thoroughly, squeezing the impregnated sheet to remove excess solution therefrom and to leave Vinsufiicient solution for binding the sheet to wood veneer, drying the impregnated sheet at temperatures not substantially above F., cutting the dried .sheet into plywood panel lengths, forming a stack of wooden cores and veneers with a cresylic acidformaldehyde aqueous binder solution therebetween, coating one side of each of a pair of sheet lengths with a cresylic acid-formaldehyde aqueous binder solution in amounts insumcient to effect a superficial water content of 10 pounds of water per 1000 square feet of coated surface, inserting the wooden stack between the coated surfaces of the pair of sheets, compressing the resulting stack and heating the same to temperatures sumcient for thermally setting the cresylic acid-formaldehyde condensation product into a hard infusible and insoluble resin thereby per manently welding each of the laminations together.

10. A hot pressed laminated product comprising a wood veneer and. covering and integrally bonded to a broad face of said veneer, a loosely matted fibrous sheet Ihaving a thermosetting resin diffused therethrough in amounts insumcient to saturate the sheet, the region of said bond having a higher concentration of resin particles than the fibrous sheet, and said sheet and resin diffused therethrough forming a smooth hard uniform nished surface completely masking the grain of the wood.

11. A hot pressed fibrous sheet covered plywood panel comprising a wood core sheet, wood veneer sheets having the grain running at an angle to the core sheet integrally bonded to the core sheet on both broad faces of the core sheet and covering and integrally bonded to a veneer sheet, a fibrous sheet having a thermosetting resin diffused therethrough in amounts insumcient to saturatethe ilbrous sheet, the region of the bond between the fibrous sheet and the veneer sheet having a higher concentration of resin particles than the fibrous sheet, and said fibrous sheet and resin diffused therethrough forming a smooth hard uniform finished surface DONALD G. BIRMINGHAM. 

